What causes these symptoms? I’m not sure, and nobody I’ve asked can tell me with total confidence, either. I checked with a couple of retired UCCE Farm Advisors with 50+ years of experience between them and, while they had seen these symptoms before, neither knew for certain what explained the yellow leaves. We all think that it might be related to water stress, perhaps linked with the 25oF swing in temperatures we saw in late May/early June (see Durham CIMIS data in the table below) that produced a lot of sunburned fruit all around the valley.

Maximum temperatures, Durham CIMIS station, 2016

These yellow leaf symptoms appear, usually, in parts of the tree canopy with low vigor, disease symptoms (cytospora canker, etc.), and blue prunes – as seen in Photo 2. From what I’ve seen, rarely does the entire tree show these symptoms, just the weaker part(s) of the canopy when sudden stress appears. The yellowing of the leaves fits with a natural plant response to water stress, not in response to a sudden toxic event like the scorched leaves near a sunburned prune (Photo 3).

Photo 3. Scorched leaves in prune attributed to a toxic event such as the nearby sunburned prune.

Since we don’t have a solid answer to the cause of the yellow prune leaves, any comments from experienced growers and PCAs to this blog post would be valuable to all.